Both silver ions and silver metal have a long history of chemical and biological activity. In particular, some of the silver salts are photoactive and can form catalytic silver nanoparticles on exposure to suitable radiation. Many chemical reactions are known to be catalyzed by both silver metal and various silver salts or silver ion complexes. Electrophilic substitutions where silver serves as a Lewis acid catalyst are also known as well as organosilver compounds with chemical reactivity.
Silver has been known for biocidal (antimicrobial) activity for over a century and renewed interest in its biocidal properties has arisen due to the emergence of antibiotic-resistant strains of pathogenic bacteria such as MERSA [see for example, Rai et al., Biotechnology Advances 27 (2009) 76-83]. There is growing evidence that silver is an effective biocide because it can attack an organism through multiple pathways and thereby disrupt multiple critical systems such as cell wall integrity and transport, protein synthesis, and DNA and RNA replication. In spite of the very aggressive attack by silver at the microbial level, it has a long history of low toxicity to humans and other complex organisms.
Silver nanoparticles can be made without polymeric stabilizers by reducing the silver ammonium complex with sugars such as glucose or maltose. Another frequently used method utilizes citrate to both complex the silver ions and to act as a reducing agent. Often an additional reducing agent such as sodium borohydride, or ascorbic acid can be used with the citrate. Often a surfactant or polymeric stabilizer such as poly(vinyl pyrrolidone) or poly(ethylene glycol (PEG) is added after silver nanoparticles are prepared without a polymeric stabilizer.
Polymer composites with silver metal are known (Review by Dallas et al., Advances in Colloid and Interface Science 166 (2011) 119-135); in commercial products such as antimicrobial coatings on medical devices such as catheters, neurological shunts (Chaloupka et al., Trends in Biotechnology, Vol. 28, No. 11, November 2010); or as coatings for contact lens storage cases (Dant m et al., Investigative Ophthalmology and Visual Science, Vol. 52, No. 1, January 2011). The silver nanoparticles can be formed by reducing silver ions usually provided as silver nitrate in the presence of a polymer than can peptize or stabilize the resulting silver nanoparticles to maintain a uniform distribution and to prevent particle agglomeration or growth through Ostwald ripening effects. A variety of polymers have been reported to serve this purpose, including poly(vinyl alcohol) or PVA copolymers, poly(vinyl pyrrolidone), and poly(ethylene glycol). A polymeric catalyst prepared by forming silver nanoparticles in modified polyethyleneimine has been reported (Signori et al., Langmuir 2010, 26(22), 17772-17779).
Efforts to use naturally occurring, biodegradable, or “green” polymers in polymer-silver composites have included starch and polysaccharides that serve as both a stabilizing agent and a reducing agent. Silver nanoparticles showing antimicrobial behavior have been formed in the presence of biodegradable and non-toxic chitosan derived from crab shells.
A large body of research has been focused on forming silver nanoparticles inside a network of water-soluble polymers that can be crosslinked to form a water-, ion-, and silver nanoparticle-permeable gel or hydrogel, for example using poly(acrylamide) (Uygun et al., Macromolecular Chemistry and Physics 2009 210, 1867-1875) or copolymers derived from an acrylamide and acrylic acid [Thomas et al., Journal of Colloid and Interface Science 315 (2007) 389-395, Mohan et al., Journal of Colloid and Interface Science 342 (2010) 73-82]. Often an additional water-soluble polymer is present during the polymerization and crosslinking of acrylamide to form an interpenetrating network (or IPN). For example, the formation of silver nanoparticles in poly(acrylamide)-based IPN's is demonstrated for poly(ethylene glycol) and poly(vinyl sulfonic acid) by Mohan et al., Journal of Colloid and Interface Science 342 (2010) 73-82, and in an acrylamide-based IPN with poly(vinylpyrrolidone) by Murthy et al., Journal of Colloid and Interface Science 318 (2008) 217-224. An acrylamide-starch IPN has been used to form silver nanoparticles for antimicrobial purposes. A hydrogel formed from vinyl caprolactam and glycidyl methacrylate has also been used to form antimicrobial gel containing silver nanoparticles. Hydrogel networks based on N-isopropylacrylamide (NIPAM) copolymerized with acrylic acid and other monomers have also shown the ability to form silver nanoparticles with antimicrobial activity while retaining the temperature responsive swelling behavior well known for such polymers.
The formation of silver nanoparticles in an ionic polymer or polyelectrolyte is also known from Girard et al, Comptes Rendus. Chimie 16(6) 550-556, 2013 where silver nanoparticles were formed in an aqueous solution of polystyrene sulfonate). Silver nanoparticles formed in poly(vinyl sulfonate) are reported in Vasilev et al., Nanotechnology 21 (2010) 215102. Hydrogels prepared using copolymers derived from 2-acrylamido-2-methylpropanesulfonate and vinyl pyrrolidone or acrylamide were loaded with preformed silver nanoparticles to evaluate the antimicrobial properties (see Valle et al., Journal of Applied Polymer Science, 2013, DOI: 10.1002/APP.38655). This art suggests that sulfonate-bearing polymers may be useful for stabilizing silver nanoparticles.
The formation of silver nanoparticles in the presence of a polymer containing both sulfonate and carboxylate groups is described in U.S. Pat. No. 8,828,275B2 (Wang et al.) where very highly concentrated dispersions are prepared for the purpose of forming a conductive silver ink. U.S. Pat. No. 8,361,553B2 (Karandikar et al.) describes polymeric silver nanoparticle dispersions formed by reducing silver ion in the presence of saccharinate derivatives and a variety of water-soluble polymers derived from both carboxylate and sulfonate bearing monomers. These dispersions are used to form antimicrobial surfaces on various substrates. Silver nanoparticle dispersions are also described in U.S. Patent Application Publication 2009/0263496A1 (Kijlstra et al.) by first forming silver oxide in the presence of various dispersing polymers such as poly(vinyl pyrrolidone), poly(aspartic acid), or poly(naphthalene sulfonic acid) and reducing the silver oxide to silver with a reducing agent such as formaldehyde.
U.S. Pat. No. 7,348,365B2 (Lee et al.) described the use of gamma radiation to reduce silver ion to silver nanoparticles in the presence of copolymers derived from vinyl pyrrolidone, acrylic acid, and acrylamide to form antimicrobial coatings. Various particles of silver salts, some specified to be less than 200 nm, are formed in the presence of water soluble polymers such as poly(vinyl pyrrolidone) and poly(acrylic acid) to form antimicrobial coatings or devices in U.S. Patent Application Publication 2008/0102122A1 (Mahadevan et al.) and U.S. Pat. No. 6,949,598B2 (Terry).
While there are numerous polymer-silver complexes described in the art, there remains a need for polymer-silver complexes that are water-soluble and water-coatable, but which can be crosslinked with UV light to become water-insoluble and highly durable after coating. There is a need for such polymer-silver complexes that can be readily used in antimicrobial compositions or articles, or that can be used to form high resolution electrically-conductive patterns without the need for added crosslinking agents or photoinitiators.
There is the further need to provide water-soluble polymeric complexes that contain reducible silver ions that are readily reduced in the polymeric complexes, before or after the polymers are crosslinked. It would be desirable to form silver nanoparticles in the size range of 1 to 500 nm using such materials.